The present invention relates to spindle-shaped goethite particles, spindle-shaped hematite particles, spindle-shaped magnetic metal particles containing iron as a main component, and processes for producing the respective particles. More particularly, the present invention relates to spindle-shaped goethite particles having an average major axial diameter of 0.05 to 0.18 xcexcm, spindle-shaped hematite particles having an average major axial diameter of 0.05 to 0.17 xcexcm, spindle-shaped magnetic metal particles containing iron as a main component, which exhibit an adequate coercive force, good dispersibility, good oxidation stability and excellent coercive force distribution notwithstanding the average major axial diameter thereof is as small as 0.05 to 0.15 xcexcm, and processes for producing the respective particles.
In recent years, recording-time prolongation, miniaturization and lightening of audio, video or computer magnetic recording and reproducing apparatuses for various magnetic recording media such as digital audio tapes (DAT) for people""s livelihood use, 8-mm video tapes, Hi-8 tapes, VTR tapes for business use, computer tapes or discs thereof have proceeded more rapidly. In particular, VTRs (video tape recorders) are now widespread, so that there have been intensely developed VTRs aiming at the transfer of analog recording types into digital ones in addition to the above recording-time prolongation, miniaturization and lightening thereof. On the other hand, with such recent tendencies, the magnetic recording media have been required to have high image quality and high output characteristics, especially high frequency characteristics. To meet these requirements, it is necessary to reduce noise due to the magnetic recording media themselves and enhance residual magnetic flux density, coercive force, dispersibility, filling property and tape-surface smoothness thereof. Therefore, it ha been further required to improve S/N ratio of the magnetic recording media.
These properties of the magnetic recording media have a close relation to magnetic particles used therein. In recent years, magnetic metal particles containing iron as a main component have been noticed because such particles can show a higher coercive force and a larger saturation magnetization as compared to those of conventional magnetic iron oxide particles, and have been already used as magnetic particles for magnetic recording media such as DAT, 8-mm video tapes, Hi-8 tapes, video tapes for business use, computer tapes or discs. The magnetic metal particles containing iron as a main component conventionally used in DAT, 8-mm video tapes, Hi-8 tapes or the like have been required to be further improved in output characteristics and weather resistance. In addition, the magnetic metal particles containing iron as a main component must fulfill applicability to existing format and good economy at the same time. Therefore, it has been strongly required to provide magnetic metal particles containing iron as a main component capable of satisfying the above requirements while minimizing amounts of various metals added thereto.
Various properties of magnetic recording media are detailed below.
In order to obtain high image quality in video magnetic recording media, it has been required to enhance S/N ratio and video frequency characteristics thereof. For this reason, it is important to improve a surface smoothness of the magnetic recording media. For improving the surface smoothness, it is also required to improve a dispersibility of magnetic particles in coating composition as well as orientation and filling properties thereof in coating film. In addition, in order to enhance the video frequency characteristics, the magnetic recording media have been required to exhibit not only a high coercive force and a large residual magnetic flux density, but also an excellent S.F.D. (Switching Field Distribution), i.e., a small coercive force distribution. Further, the magnetic recording media are required to show good running property upon repeated use, good still property as well as high recording reliability even when used under severe environmental conditions, i.e., high durability.
As to the magnetic metal particles containing iron as a main component for magnetic recording media capable of satisfying the above various properties, those having a larger particle size are preferable from the standpoint of improvement in dispersibility and oxidation stability, while those having a smaller particle size are preferable from the standpoint of improvement in surface smoothness and reduction in noise. Thus, the smaller the particle size of the magnetic metal particles containing iron as a main component, the poorer the dispersibility and oxidation stability thereof. Also, when the particle size becomes smaller, the coercive force is usually increased. Therefore, it is necessary to appropriately control the particle size for attaining aimed magnetic properties. Further, it is preferable to incorporate a large amount of cobalt into the magnetic metal particles containing iron as a main component in the consideration of chemical composition thereof since as well known, which cobalt forms a solid solution with iron and contributes to improvement in oxidation stability. However, the use of a large amount of expensive cobalt is disadvantageous from economical viewpoint. Consequently, it has been demanded to provide magnetic metal particles containing iron as a main component exhibiting an adequate coercive force as well as excellent dispersibility and oxidation stability in spite of lessening contents of expensive metal elements such as cobalt and reducing the particle size.
As known in the arts, the magnetic metal particles containing iron as a main component are produced by heat-treating goethite particles, hematite particles obtained by heat-dehydrating the goethite particles, or particles obtained by incorporating different metal elements into the above goethite or hematite particles as starting material, if required, in a non-reducing atmosphere; and then heat-reducing the resultant particles in a reducing atmosphere. In this case, it is required that the magnetic metal particles containing iron as a main component still maintain shape and size of the goethite particles as starting material by appropriately controlling the shape and size of the goethite particles or by preventing heat fusion between particles upon heat-treatments such as heat-dehydration and heat-reduction, or deformation and breakage of each particle.
The starting goethite particles are classified into two kinds of goethite particles in accordance with configurations thereof, i.e., acicular goethite particles produced mainly from alkali hydroxide and spindle-shaped goethite particles produced mainly from alkali carbonate. The acicular goethite particles usually tend to have a large aspect ratio, but tend to be deteriorated in particle size distribution and become large in size as compared to spindle-shaped goethite particles. The particle size distribution is an index of uniformity of primary particles, and, therefore, has a close relationship with coercive force distribution and oxidation stability of the magnetic metal particles containing iron as a main component. Consequently, spindle-shaped goethite particles having an excellent particle size distribution are preferably used as the starting materials of the magnetic metal particles containing iron as a main component.
Under these circumstances, as magnetic metal particles containing iron as a main component used for audio or video magnetic recording media such as digital audio tapes (DAT) for people""s livelihood use, 8-mm video tapes and Hi-8 tapes, there have been demanded such magnetic metal particles containing iron as a main component having an adequate coercive force of 111.4 to 143.2 kA/m (1,400 to 1,800 Oe) as well as good dispersibility and oxidation stability even when the content of expensive elements such as cobalt is lessened and the particle size is minimized in order to further improve the properties of the magnetic metal particles containing iron as a main component and pursue better economy.
As to compositions of magnetic metal particles containing iron as a main component for improving various properties thereof, there are known those specified in, e.g., Japanese Patent Application Laid-Open (KOKAI) Nos. 7-210856, 8-279142, 9-293233, 9-295814, 10-69629, 10-245233, 10-275326, 10-334455, 10-334457, 11-11951, 11-130439, 11-251122 or the like.
At present, it has been strongly required to provide spindle-shaped magnetic metal particles containing iron as a main component, which can exhibit an adequate coercive force of 111.4 to 143.2 kA/m (1,400 to 1,800 Oe), good dispersibility, good oxidation stability and excellent coercive force distribution in spite of fine particles, especially notwithstanding the average major axial diameter thereof is as small as 0.05 to 0.15 xcexcm. However, such spindle-shaped magnetic metal particles containing iron as a main component cannot be obtained yet.
Namely, in the above-described KOKAIs, although the contents of Co, Al and rare earth element based on whole Fe are specified, there are no descriptions concerning relationship between the respective elements. These conventional despite fail to satisfy the above requirement of exhibiting an adequate coercive force, excellent dispersibility and excellent oxidation stability at the same time in spite of fine particles.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that in the process for producing spindle-shaped goethite particles, comprising reacting an aqueous ferrous salt solution with a mixed aqueous alkali solution of an aqueous alkali carbonate solution and an aqueous alkali hydroxide solution; aging the resultant water suspension containing a ferrous-containing precipitate in a non-reducing atmosphere; passing an oxygen-containing gas through the water suspension to conduct an oxidation reaction thereof, thereby producing spindle-shaped goethite seed crystal particles; passing an oxygen-containing gas through the water suspension containing the spindle-shaped goethite seed crystal particles and the ferrous-containing precipitate to conduct an oxidation reaction thereof, thereby growing a goethite layer on the surface of each spindle-shaped goethite seed crystal particle, so that spindle-shaped goethite particles are obtained; and heat-treating the resultant spindle-shaped goethite particles in a non-reducing atmosphere; and then heat-reducing the obtained particles in a reducing atmosphere,
by adding a Co compound in an amount of from 0.5 to less than 6 atm % (calculated as Co) based on whole Fe, to the water suspension containing the ferrous-containing precipitate during aging but prior to the elapse of half a period of the whole aging time before initiation of the oxidation reaction; conducting the oxidation reaction for producing the spindle-shaped goethite seed crystal particles, such that 40 to 50% of whole Fe2+ is oxidized; adding an Al compound in an amount of from more than 10 to less than 20 atm % (calculated as Al) based on whole Fe, to the water suspension containing the spindle-shaped goethite seed crystal particles and ferrous-containing precipitate before initiation of the oxidation reaction,
the thus obtained spindle-shaped magnetic metal particles containing iron as a main component can exhibit not only adequate coercive force, good dispersibility and good oxidation stability, but also excellent coercive force distribution notwithstanding the average major axial diameter thereof is as small as 0.05 to 0.15 xcexcm. The present invention has been attained on the basis of this finding.
An object of the present invention is to provide fine spindle-shaped magnetic metal particles containing iron as a main component, which can exhibit not only adequate coercive force, good dispersibility and good oxidation stability, but also excellent coercive force distribution in spite of the average major axial diameter thereof of as small as 0.05 to 0.15 xcexcm.
Another object of the present invention is to provide fine spindle-shaped hematite particles having an average major axial diameter of 0.05 to 0.17 xcexcm and a specific crystallite size ratio, which are suitably used as a starting material of the fine spindle-shaped magnetic metal particles containing iron as a main component which can exhibit not only adequate coercive force, good dispersibility and good oxidation stability, but also excellent coercive force distribution.
Still another object of the present invention is to provide fine spindle-shaped goethite particles having an average major axial diameter of 0.05 to 0.18 xcexcm, a relatively small aspect ratio and a specific crystallite size ratio, which are suitably used as a starting material of the fine spindle-shaped magnetic metal particles containing iron as a main component which can exhibit not only adequate coercive force, good dispersibility and good oxidation stability, but also excellent coercive force distribution.
To accomplish the aim, in a first aspect of the present invention, there are provided spindle-shaped goethite particles having an average minor axial diameter of 0.05 to 0.18 xcexcm, an aspect ratio of from more than 6:1 to less than 10:1, a size distribution (standard deviation/average major axial diameter) of not more than 0.20, a Co content of from 0.5 to less than 6 atm % based on whole Fe; an Al content of from more than 10 to less than 20 atm % based on whole Fe, and an atomic ratio of Al to Co of from more than 2 to 4.
In a second aspect of the present invention, there are provided spindle-shaped goethite particles having an average major axial diameter of 0.05 to 0.18 xcexcm, an aspect ratio of from more than 6:1 to less than 10:1, a size distribution (standard deviation/average major axial diameter) of not more than 0.20, a Co content of from 0.5 to less than 5 atm % based on whole Fe; an Al content of from 10.5 to less than 18 atm % based on whole Fe, an atomic ratio of Al to Co of from 2.10 to 3.90, a crystallite size D020 of 150 to 250 xc3x85, a crystallite size D110 of 80 to 120 xc3x85 and a crystallite size ratio of D020/D110 of 1.8 to 2.4.
In a third aspect of the present invention, there are provided spindle-shaped hematite particles having an average major axial diameter of 0.05 to 0.17 xcexcm, an aspect ratio of from more than 6:1 to less than 10:1, a size distribution (standard deviation/average major axial diameter) of not more than 0.22, a crystallite size D104 of 100 to 140 xc3x85, a Co content of from 0.5 to less than 6 atm % based on whole Fe; an Al content of from more than 10 to less than 20 atm % based on whole Fe, a rare earth element content of from 1.5 to 5 atm % based on whole Fe, and an atomic ratio of Al to Co of from more than 2 to 4.
In a fourth aspect of the present invention, there are provided spindle-shaped hematite particles having an average major axial diameter of 0.05 to 0.17 xcexcm, an aspect ratio of from more than 6:1 to less than 10:1, a size distribution (standard deviation/average major axial diameter) of not more than 0.22, a crystallite size D104 of 100 to 140 xc3x85, a Co content of from 0.5 to less than 5 atm % based on whole Fe; an Al content of from 10.5 to less than 18 atm % based on whole Fe, a rare earth element content of from 2.0 to 4.8 atm % based on whole Fe, an atomic ratio of Al to Co of from 2.10 to 3.90, a crystallite size D110 of 200 to 300 xc3x85, and a crystallite size ratio of D020/D110 of 2.0 to 4.0.
In a fifth aspect of the present invention, there are provided spindle-shaped magnetic metal particles containing iron as a main component, having an average major axial diameter of 0.05 to 0.15 xcexcm, an aspect ratio of from 5:1 to 9:1, a size distribution (standard deviation/average major axial diameter) of not more than 0.30, a crystallite size D110 of 130 to 160 xc3x85, a Co content of from 0.5 to less than 6 atm % based on whole Fe, an Al content of from more than 10 to less than 20 atm % based on whole Fe, a rare earth element content of from 1.5 to 5 atm % based on whole Fe, an atomic ratio of Al to Co of from more than 2 to 4, a coercive force of 111.4 to 143.2 kA/m, an oxidation stability of saturation magnetization (xcex94"sgr"s) of not more than 10%, and an ignition temperature of not less than 130xc2x0 C.
In a sixth aspect of the present invention, there are provided spindle-shaped magnetic metal particles containing iron as a main component containing iron as a main component, having an average major axial diameter of 0.05 to 0.15 xcexcm, an aspect ratio of from 5:1 to 9:1, a size distribution (standard deviation/average major axial diameter) of not more than 0.30, a crystallite size D110 of 130 to 160 xc3x85, a Co content of from 0.5 to less than 5 atm % based on whole Fe; an Al content of from 10.5 to less than 18 atm % based on whole Fe, a rare earth element content of from 2.0 to 4.8 atm % based on whole Fe, an atomic ratio of Al to Co of from 2.10 to 3.90, a coercive force of 111.4 to 143.2 kA/m, an oxidation stability of saturation magnetization (xcex94"sgr"s) of not more than 10%, and an ignition temperature of not less than 130xc2x0 C.
In a seventh aspect of the present invention, there is provided a process for producing the spindle-shaped goethite particles, which process comprises:
aging a water suspension containing a ferrous-containing precipitate produced by reacting a mixed aqueous alkali solution comprising an aqueous alkali carbonate solution and an aqueous alkali hydroxide solution with an aqueous ferrous salt solution in a non-oxidative atmosphere;
conducting an oxidation reaction of the water suspension by passing an oxygen-containing gas therethrough, thereby producing spindle-shaped goethite seed crystal particles,
upon the production of the seed crystal particles, a Co compound being added in an amount of from 0.5 to less than 6 atm %, calculated as Co, based on whole Fe, to the water suspension containing the ferrous-containing precipitate during aging thereof but prior to the elapse of half a period of the whole aging time before initiation of the oxidation reaction, and the oxidation reaction being conducted such that 40 to 50% of whole Fe2+ is oxidized; and
after adding an Al compound in an amount of from more than 10 to less than 20 atm %, calculated as Al, based on a whole Fe, to the water suspension containing both the ferrous-containing precipitate and the spindle-shaped goethite seed crystal particles, passing again an oxygen-containing gas through the resultant water suspension so as to conduct an oxidation reaction thereof, thereby growing a goethite layer on the surface of each seed crystal particle.
In an eighth aspect of the present invention, there is provided a process for producing the spindle-shaped hematite particles, which process comprises:
aging a water suspension containing a ferrous-containing precipitate produced by reacting a mixed aqueous alkali solution comprising an aqueous alkali carbonate solution and an aqueous alkali hydroxide solution with an aqueous ferrous salt solution in a non-oxidative atmosphere;
conducting an oxidation reaction of the water suspension by passing an oxygen-containing gas therethrough, thereby producing spindle-shaped goethite seed crystal particles,
upon the production of the seed crystal particles, a Co compound being added in an amount of from 0.5 to less than 6 atm %, calculated as Co, based on whole Fe, to the water suspension containing the ferrous-containing precipitate during aging thereof but prior to the elapse of half a period of the whole aging time before initiation of the oxidation reaction, and the oxidation reaction being conducted such that 40 to 50% of whole Fe2+ is oxidized;
after adding an Al compound in an amount of from more than 10 to less than 20 atm %, calculated as Al, based on a whole Fe, to the water suspension containing both the ferrous-containing precipitate and the spindle-shaped goethite seed crystal particles, passing again an oxygen-containing gas through the resultant water suspension so as to conduct an oxidation reaction thereof, thereby growing a goethite layer on the surface of each seed crystal particle;
treating the spindle-shaped goethite particles obtained with an anti-sintering agent containing a rare earth compound in an amount of 1.5 to 5 atm %, calculated as a rare earth element, based on whole Fe; and
heat-treating the thus treated spindle-shaped goethite particles at a temperature of 650 to 800xc2x0 C.in a non-reducing atmosphere.
In a ninth aspect of the present invention, there is provided a process for producing the spindle-shaped magnetic metal particles containing iron as a main component, which process comprises:
aging a water suspension containing a ferrous-containing precipitate produced by reacting a mixed aqueous alkali solution comprising an aqueous alkali carbonate solution and an aqueous alkali hydroxide solution with an aqueous ferrous salt solution in a non-oxidative atmosphere;
conducting an oxidation reaction of the water suspension by passing an oxygen-containing gas therethrough, thereby producing spindle-shaped goethite seed crystal particles,
upon the production of the seed crystal particles, a Co compound being added in an amount of from 0.5 to less than 6 atm %, calculated as Co, based on whole Fe, to the water suspension containing the ferrous-containing precipitate during aging thereof but prior to the elapse of half a period of the whole aging time before initiation of the oxidation reaction, and the oxidation reaction being conducted such that 40 to 50% of whole Fe2+ is oxidized;
after adding an Al compound in an amount of from more than 10 to less than 20 atm %, calculated as Al, based on a whole Fe, to the water suspension containing both the ferrous-containing precipitate and the spindle-shaped goethite seed crystal particles, passing again an oxygen-containing gas through the resultant water suspension so as to conduct an oxidation reaction thereof, thereby growing a goethite layer on the surface of each seed crystal particle;
treating the spindle-shaped goethite particles obtained with an anti-sintering agent containing a rare earth compound in an amount of 1.5 to 5 atm %, calculated as a rare earth element, based on whole Fe;
heat-treating the thus treated spindle-shaped goethite particles at a temperature of 650 to 800xc2x0 C. in a non-reducing atmosphere;
heat-reducing the spindle-shaped hematite particles obtained at 400 to 700xc2x0 C. in a reducing atmosphere.
In a tenth aspect of the present invention, there is provided a magnetic recording medium comprising a non-magnetic substrate and a magnetic recording layer formed on the non-magnetic substrate comprising a binder resin and spindle-shaped magnetic metal particles containing iron as a main component, which have an average major axial diameter of 0.05 to 0.15 xcexcm, an aspect ratio of 5:1 to 9:1, a size distribution (standard deviation/average major axial diameter) of not more than 0.30, a crystallite size D110 of 130 to 160 xc3x85, a Co content of from 0.5 to less than 6 atm % based on whole Fe; an Al content of from more than 10 to less than 20 atm % based on whole Fe, a rare earth element content of 1.5 to 5 atm % based on whole Fe, an atomic ratio of Al to Co of from more than 2 to 4, a coercive force of 111.4 to 143.2 kA/m, an oxidation stability of saturation magnetization (xcex94"sgr"s) of not more than 10%, and an ignition temperature of not less than 130xc2x0 C.